1. Field of the Invention
This invention relates to a communication receiver.
2. Description of Related Art
The European DVB-T (Digital Video Broadcasting-Terrestrial) standard for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore grouped into blocks and frames.
After reception in the television receiver, the signals are sampled, for example using a resampler, and are mixed down to baseband. The start of each active symbol is found, and then the active symbols are applied to a Fast Fourier Transform (FFT) processor, and subsequently to a channel estimator, to extract the wanted information.
It is necessary to transmit the DTT signals over transmission paths which are of uncertain quality. In particular, the area close to the transmission path may include objects such as tall buildings, which cause echoes. That is, a signal may be received at a receiver twice, once on a direct path from the transmitter, and then, after a short delay, as an echo. Further, there may be no direct line of sight from the transmitter to the receiver, in which case the receiver will only receive echoes. The effect of this is that the first signal received may not necessarily have the strongest power. There will therefore be combinations of pre-echoes arriving before the strongest signal and echoes arriving afterwards.
As is well known, this scenario can cause inter-symbol interference (ISI) in the receiver. To reduce the effects associated with this problem, DVB-T COFDM signals include a cyclic prefix guard interval for each active symbol. Specifically, a portion of the active symbol is repeated before the next active symbol.
Once the received signal is converted down to baseband, if there is a large echo present, a time domain correlation between samples which are an active window length apart yield large powers in the guard interval of the echo. These correlations can be used to correctly position the window when large echoes are present, although the technique is not as effective for smaller echoes. If the smaller echoes lag the larger ones, then correct positioning of the windowing relative to the first large echo (or relative to the main signal if no large pre-echo is present), will result in a good solution. On the other hand, if the smaller echo is a pre-echo, this may not be the case, as the pre-echo will be introducing ISI.
One solution to this problem is to pull back the window position, calculated using the correlations in time, which can avoid ISI, but which rotates the signal in the frequency domain. Large rotations in the frequency domain can adversely affect the performance of the channel estimator. Moreover, the guard interval prefix must be removed before the signals are further processed. The initial position of the prefix can be found, and it is also preferable to allow correction for any changes in position caused by subsequent variations in sampling rate. Again such corrections have the effect of rotating the signal in the frequency domain.